1. Field of the Invention
The present invention relates to an EL (electroluminescence) display device obtained by fabricating semiconductor elements (elements formed by using a thin semiconductor film) on a substrate and to an electronic device having the EL display device as a display unit.
2. Prior Art
Technology has been greatly advanced in recent years for forming TFTs on a substrate, and attempts have been made to develop an active matrix-type display device. In particular, TFTs using a polysilicon film exhibit a higher electric-field mobility (also called mobility) than that of the conventional TFTs using an amorphous silicon film, and make it possible to accomplish a high-speed operation. This makes it possible to control the pixels, which has heretofore been done by a drive circuit outside the substrate, by using a drive circuit formed on the same substrate as the pixels.
The active matrix-type display device is drawing attention owing it its advantages such as a decrease in the cost of production, a decrease in the size of the display device, an increase in the yield and a decrease in the throughput, as a result of fabricating various circuits and elements on the same substrate.
The active matrix-type EL display devices have heretofore been employing pixels of a structure that is generally as shown in FIG. 3. In FIG. 3, reference numeral 301 denotes a TFT (hereinafter referred to as switching TFT) that works as a switching element, 302 denotes a TFT (hereinafter referred to as current control TFT) working as an element (current control element) for controlling the current supplied to an EL element 303, and 304 denotes a capacitor (holding capacity). The switching TFT 301 is connected to a gate wiring 305 and to a source wiring (data wiring) 306. The drain of the current control TFT 302 is connected to the EL element 303, and the source thereof is connected to a current feeder line 307.
When the gate wiring 305 is selected, the gate of the switching TFT 301 is opened, a data signal of the source wiring 306 is accumulated in the capacitor 304, and the gate of the current control TFT 302 is opened. After the gate of the switching TFT 301 is closed, the gate of the current control TFT 302 remains opened due to the electric charge accumulated in the capacitor 304 and, during this period, the EL element 303 emits light. The amount of light emitted by the EL element 303 varies depending on the amount of electric current that flows.
Here, the amount of current supplied to the EL element 303 is controlled by a gate voltage of the current control TFT 302 as shown in FIG. 4.
FIG. 4(A) is a graph illustrating transistor characteristics of the current control TFT, wherein a curve 401 represents Id-Vg characteristics (or an Id-Vg curve), Id represents a drain current and Vg represents a gate voltage. From this graph, it is possible to know the amount of current that flows relative to any gate voltage.
Usually, the EL element is driven by utilizing the Id-Vg characteristics over a region surrounded by a dotted line 402. FIG. 4(B) is a view illustrating the region surrounded by 402 on an enlarged scale.
In FIG. 4(B), the hatched region is called sub-threshold region. In practice, this region has a gate voltage close to or lower than the threshold voltage (Vth) and where the drain current exponentially varies depending upon a change in the gate voltage. In this region, the current is controlled based on the gate voltage.
The data signal input to the pixel as the switching TFT 301 is opened is, first, accumulated in the capacitor 304 and directly serves as a gate voltage for the current control TFT 302. Here, the drain current is determined for the gate voltage at a ratio of 1 to 1 in compliance with the Id-Vg characteristics shown in FIG. 4(A). That is, a predetermined current flows through the EL element 303 depending on the data signal, and the EL element 303 emits light in an amount corresponding to the amount of current.
Thus, the amount of light emitted by the EL element is controlled by the data signal, and the gradation display is accomplished by controlling the amount of light that is emitted. This system is a so-called analog gradation; i.e., the gradation display is accomplished relying on a change in the amplitude of the signal.
However, the analog gradation system has a defect that it is very susceptible to dispersion in the characteristics of the TFTs. For example, considered below is a case where a switching TFT that would exhibit the same gradation has ID-Vg characteristics different from those of a switching TFT of the neighboring pixel (generally shifted toward the positive side or the negative side).
In this case, the drain currents flowing into the switching TFTs vary depending upon the degree of dispersion, and different gate voltages are applied to the current control TFTs of the pixels. That is different currents flow into the EL elements and, as a result, light is emitted in different amounts making it no longer possible to accomplish the same gradation display.
Further, even when the same gate voltage is applied to the current control TFTs of the pixels, the same drain current cannot be produced if there is a dispersion in the Id-Vg characteristics of the current control TFTs. As will be obvious from FIG. 4(A), further, since use is made of the region where the drain current exponentially changes relative to the change in the gate voltage, even a slight difference in the Id-Vg characteristics results in a large change in the amount of current despite an equal gate voltage is applied. Then, the amount of light emitted by the EL elements greatly varies among the neighboring pixels.
In practice, the situation becomes more severe due to synergistic effect of dispersion of both the switching TFTs and the current control TFTs. Thus, the analog gradation system is very susceptible to the dispersion in the characteristics of the TFTs, hindering the attempt for realizing the multi-color active matrix EL display device.
The present invention was accomplished in view of the above-mentioned problems, and provides an active matrix-type EL display device capable of producing a vivid multi-gradation color display. The invention further provides an electronic device of high performance using the active matrix-type EL display device as a display unit.
The present applicant has discovered the fact that the problems of the analog gradation system stem from the dispersion in the characteristics of the current control TFTs that control the current flowing into the EL elements and from the dispersion in the on-resistance of the current control TFTs. Here, the on-resistance is a value obtained by dividing the drain voltage of the TFT by the drain current flowing at that moment.
That is, the on-resistance varies among the current control TFTs and, hence, different currents (drain currents) flow even under the same condition, making it difficult to obtain a desired gradation.
According to this invention, therefore, a resistor (R) is connected in series between the drain of the current control TFT and the EL element to control the amount of current supplied from the current control TFT to the EL element. For this purpose, it is necessary to provide a resistor having a resistance very larger than the on-resistance of the current control TFT. The resistance may be selected over a range of from 1 kxcexa9 to 50 Mxcexa9 (preferably, from 10 kxcexa9 to 10 Mxcexa9 and, more preferably, from 50 kxcexa9 to 1 Mxcexa9).
In carrying out the invention, further, the amount of current flowing into the EL element is determined by the resistance of the resistor (R), and the supplied current becomes constant at all times. That is, the invention does not use the analog gradation system that produces the gradation display by controlling the current value that is done by the prior art. The invention therefore uses the gradation display of the time-division system (hereinafter referred to as time-division gradation) using the current control TFTs simply as switching elements for supplying electric current.
Concretely speaking, the time-division gradation display is accomplished in a manner as described below. Described here is a case of full-color display of 256 gradations (16,770,000 colors) based on the 8-bit digital drive system.
First, a picture frame is divided into eight sub-frames. Here, a period for inputting data to all pixels of the display region is called a frame. In an ordinary EL display, the oscillation frequency is 60 Hz, i.e., 60 frames are formed in a second. When the number of frames per a second becomes smaller than this value, flickering of the picture becomes conspicuous. Further, frames divided into a plural number from a frame are called sub-frames.
Each sub-frame can be divided into an address period (Ta) and a sustain period (Ts). The address period stands for a time necessary for inputting data to all pixels during a sub-frame period, and the sustain period (or turn-on period) stands for a period in which the EL element emits light (FIG. 10).
Here, a first sub-frame is denoted by SF1, and the second sub-frame to the eighth sub-frame are denoted by SF2 to SF8. The address period (Ta) remains constant from SF1 through up to SF8. The sustain periods (Ts) of SF1 to SF8 are denoted by Ts1 to Ts8.
At this moment, the sustain periods are so set that Ts1:Ts2:Ts3:Ts4:Ts5:Ts6:Ts7:Ts8=1:1/2:1/4:1/8:1/16:1/32:1/64:1/128. Here, SF1 to SF8 may appear in any order. Any desired gradation display can be accomplished out of 256 gradations relying on the combinations of the sustain periods.
First, no voltage (for not to select) is applied to the opposing electrode (the one of the side not connected to the TFT, and is usually a cathode) of the EL element of the pixel, and the data signal is input to all of the pixels without causing EL elements to emit light. This period is an address period. When the data is input to all pixels to end the address period, a voltage is applied to the opposing electrodes (selected) so that the EL elements emit light simultaneously. This period is a sustain period. The period for emitting light (for turning the pixel on) is any one of the periods Ts1 to Ts8. It is, here, presumed that a predetermined pixel is turned on for the period Ts8.
The time enters into the address period again and, then, enters into the sustain period after the data signal is input to all pixels. In this case, any one of Ts1 to Ts7 is the sustain period. Here, a predetermined pixel is turned on for the period Ts7.
Hereinafter, the same operation is repeated for the remaining six sub-frames, the sustain periods are successively set like Ts6, Ts5, . . . , Ts1, and the predetermined pixels are turned on in the respective sub-frames.
When eight sub-frames have appeared, it means an end of a frame. In this case, the gradation of the pixel is controlled by adding up the sustain periods. When, for example, Ts1 and Ts2 are selected, the luminance of 75% can be expressed out of the total light of 100%. When Ts3, Ts5 and Ts8 are selected, 16% of the luminance can be expressed.
In the foregoing was described the case of 256 gradations. It is, however, also allowable to effect any other gradation display.
To effect the display of a gradation (2n gradation) of n bits (n is an integer of not smaller than 2), first, a frame is divided into n sub-frames (SF1, SF2, SF3, . . . , SF(nxe2x88x921), SF(n)) to correspond to the gradation of n bits. As the gradation increases, the frame must be divided into an increased number, and the drive circuits must be driven at a high frequency.
These n pieces of sub-frames are separated into address periods (Ta) and sustain periods (Ts). That is, the address periods and the sustain periods are selected by either applying or not applying a voltage to the opposing electrode common to all EL elements.
The sustain periods (the sustain periods corresponding to SF1, SF2, SF3, . . . , SF(nxe2x88x921), SF(n)) of n sub-frames are so processed that Ts1:Ts2:Ts3: . . . :Ts(nxe2x88x921):Ts(n)=20:2xe2x88x921:2xe2x88x922: . . . :2xe2x88x92(nxe2x88x922):2xe2x88x92(nxe2x88x921).
In this state, the pixels are successively selected in any sub-frame (strictly, switching TFTs of the pixels are selected), and a predetermined gate voltage (corresponding to data signal) is applied to the gate electrodes of the current control TFTs. At this moment, the EL element of the pixel that has received a data signal which renders the current control TFT to be conductive, emits light for the sustain period assigned to the sub-frame after the address period has been finished. That is, a predetermined pixel is turned on.
This operation is repeated for all n sub-frames to control the gradation of the pixels by adding up the sustain periods. Therefore, if attention is given to a certain pixel, the gradation of the pixel is controlled depending upon how long period the pixel is turned on by the sub-frames (depending upon how many sustain periods it has passed through).
As described above, the feature of the present invention resides on executing the time-division gradation display by using the active matrix-type EL display device, and by providing a resistor (R) between the drain of the current control TFT and the EL element to set constant the current that flows through the EL element at all times. This constitution makes it possible to prevent defect in the gradation caused by dispersion in the characteristics of the TFTs.